(a) Field of the Invention
The present invention relates to a liquid crystal display and a driving method thereof. More particularly, the present invention relates to a field sequential driving type liquid crystal display (FS-LCD) and a driving method thereof.
(b) Description of the Related Art
As personal computers and televisions, etc., have become more lightweight and thin, the demand for lightweight and thin display devices has increased. According to such requirements, flat panel displays such as liquid crystal displays (LCD) have recently been developed instead of cathode ray tubes (CRT).
An LCD is a display device used to display a desired video signal by applying electric fields to liquid crystal materials having an anisotropic dielectric constant and injected between two substrates, and controlling the strength of electric fields so as to control an amount of light from an external light source (i.e., backlight) transmitted through a substrate.
The LCD is representative of portable flat panel displays, and TFT-LCDs using a thin film transistor (TFT) as a switching element are mainly used.
Each pixel in the TFT-LCD can be modeled with capacitors having liquid crystal as a dielectric substance, such as a liquid crystal capacitor. An equivalent circuit of each pixel in such an LCD is as shown in FIG. 1.
As shown in FIG. 1, each pixel of a liquid crystal display includes a TFT 10, of which a source electrode and a gate electrode are respectively connected to a data line (Dm) and a scanning line (Sn); a liquid crystal capacitor Cl connected between a drain electrode of the TFT and common voltage Vcom; and a storage capacitor Cst connected to the drain electrode of the TFT.
In FIG. 1, when a scanning signal is applied to a scanning line (Sn) and the TFT 10 is turned on, data voltages (Vd) supplied to the data line are applied to each pixel electrode (not shown) though the TFT. Then, an electric field corresponding to a difference between pixel voltages Vp applied to pixel electrodes and the common voltage Vcom is applied to liquid crystal (which is equivalently shown as the liquid crystal capacitor Cl in FIG. 1). Light transmits with a transmittivity corresponding to the strength of the electric field. In this instance, a pixel voltage Vp needs to be maintained during one frame or one field, so the storage capacitor Cst in FIG. 1 is used to maintain a pixel voltage Vp applied to a pixel electrode.
Generally, liquid crystal display can be classified into two methods, a color filter method and a field sequential driving method, based on methods of displaying color images.
A liquid crystal display of a color filter method has color filter layers composed of three primary colors such as red R, green G, and blue B in one of two substrates, and displays a desired color by controlling an amount of light transmitted through the color filter layer. A liquid crystal display of a color filter method controls an amount of light transmitted through the R, G, and B color filter layers when light from a single light source transmits through the R, G, and B color filter layers, and composes R, G, and B colors to display a desired color.
A liquid crystal display device displaying color using a single light source and 3 color filter layers needs unit pixels respectively corresponding to each R, G, and B subpixel, thus at least 3 times the number of pixels are needed compared with displaying black and white. Therefore, fine manufacturing techniques are required to produce video images of high definition.
Further, there are problems in that separate color filter layers must be formed on a substrate for a liquid crystal display in manufacturing, and the light transmission rate of the color filters must be improved.
On the other hand, a field sequential driving type of liquid crystal display sequentially and periodically turns on each independent light source of R, G, and B colors, and adds synchronized color signals corresponding to each pixel based on the lighting periodic time to obtain full colors. That is, according to a field sequential driving type of liquid crystal display, one pixel is not divided into R, G, and B subpixels, and light of 3 primary colors outputted from R, G, and B back lights is sequentially displayed in a time-divisional manner so that the color images are displayed using an after image effect of the eye.
The field sequential driving method can be classified as an analog driving method and a digital driving method.
The analog driving method establishes a plurality of gray voltages, selects one gray voltage corresponding to gray data from among the gray voltages, and drives a liquid crystal panel with the selected gray voltage to perform gray display with an amount of transmission corresponding to the gray voltage applied.
FIG. 2 shows a driving voltage and amount of light transmission of a conventional liquid crystal display of the analog driving method.
In FIG. 2, the driving voltage is a voltage applied to liquid crystal, and optical transmittivity is transmittivity through the liquid crystal. That is, optical transmittivity refers to a torsion degree of the liquid crystal that allows light to transmit.
Referring to FIG. 2, a driving voltage having a V11 level is applied to the liquid crystal, and light corresponding to the driving voltage having the V11 level transmits through the liquid crystal in the R field period Tr for displaying an R color. A driving voltage having a V12 level is applied to the liquid crystal, and light corresponding to the driving voltage having the V12 level transmits through the liquid crystal in the G field period Tg for displaying a G color. Further, a V13 level driving voltage is applied to the liquid crystal, and an amount of light transmission corresponding to the V13 level is obtained. A desired color image is displayed by combination of R, G, and B lights transmitted respectively during Tr, Tg, and Tb periods.
On the other hand, a digital driving method applies a constant driving voltage to the liquid crystal, and controls the voltage applying time to perform a gray display. The digital driving method maintains a constant driving voltage, and controls timing of a voltage applying state and a voltage non-applying state, so as to control a total amount of light transmitting through the liquid crystal.
FIG. 3 shows a waveform which illustrates a driving method of a liquid crystal display of a conventional digital driving method, and shows a waveform of a driving voltage and optical transmittivity of liquid crystal based on driving data of a predetermined bit.
Referring to FIG. 3, gray waveform data corresponding to each gray is provided with a digital signal having a predetermined number of bits, for example a 7 bit digital signal, and a gray waveform according to 7 bit data is applied to the liquid crystal. Optical transmittivity of the liquid crystal is determined based on the gray waveform applied to perform gray display.
In the conventional field sequential driving method, correct gray is typically not displayed since an effective value response of a desired gray for display (for example, a gray scale of R) is changed by a previous gray display (for example, a gray of G). That is, a pixel voltage Vp actually applied to the liquid crystal is determined by a gray voltage (or a gray waveform) supplied to a present field (for example, an R field) and a gray voltage (or a gray waveform) supplied to the previous field (for example, a B field).
U.S. Pat. No. 6,567,063 (“the '063 patent”) discloses a field sequential driving method using a reset pulse to solve the problem of the field sequential driving method in which an effective value response of the desired gray is changed because of a previous gray display.
FIG. 4 shows a field sequential driving method using a reset pulse described in the '063 patent. In FIG. 4, periods (T31˜T36) indicate an R field, a G field, and a B field performing gray display for each of R, G, and B.
Referring to FIG. 4, a predetermined voltage (reset voltage) is applied, which is independent of input gray data, and is more than a maximum value of gray data applied during a predetermined time (t31˜t36) at the point where each of the periods (T31˜T36) is ended. A state of all the liquid crystals is reset to the same state (for example, a black state in which no light can be transmitted, that is, optical transmittivity is 0) at the point where each of the periods (T31˜T36) is ended.
Thus, when the liquid crystals are driven by voltages applied with gray data at each period (T31˜36), the state of the liquid crystals become the same regardless of previous grays displayed, thus the display period for the present gray is not affected by the previous gray display.
However, according to the '063 patent, since a reset voltage of a constant size and width of more than a maximum value of gray data is always applied regardless of input gray data, there is a problem in that power consumption is increased.